For how many different negative values of $x$ is $\sqrt{x +150}$ a positive integer?
Let $\sqrt{x + 150} = n$, where $n$ is a positive integer.  Then $x + 150 = n^2$, so $x = n^2 - 150$.  We see that $x$ is negative for $n = 1$, 2, 3, $\dots$, 12, but positive for $n \ge 13$, so the number of possible values of $x$ is $\boxed{12}$.